Computers and related devices are increasingly being connected into networks for communications between the devices. Typically, the networks comprise LANs (local area networks) which provide communications among devices within a relatively small geographical area different LANs being interconnected via MANs (metropolitan area networks) and WANs (wide area networks). This has resulted in a global computer information network which is generally known as the Internet. The term xe2x80x9cNetworkxe2x80x9d is used herein to refer generically to this global computer information network and to any other network of computers and related devices.
Different technologies can be used to facilitate communications on any LAN and throughout the Network, the most common being Carrier Sense Multiple Access with Collision Detection (CSMA/CD) technology. This is documented in IEEE Standard 802.3 entitled xe2x80x9cCarrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specificationsxe2x80x9d which has been adopted by the International Organization for Standardization (ISO). The 802.3 Standard is based on the 1985 Version 2 Standard for Ethernet and, although there are some differences including different use of a length/type field, the two Standards are largely interchangeable and can be considered equivalent as far as this invention is concerned. The term xe2x80x9cCSMA/CDxe2x80x9d is used herein to refer generically to this technology. Using CSMA/CD, packets of data are communicated in frames that are generally referred to as Ethernet frames. This term is also used herein, regardless of whether the frames comply with the 802.3 Standard or the Ethernet Standard (i.e. regardless of the value contained in the length/type field of the frame).
The OSI (Open Systems Interconnection) reference model established by the ISO defines packetized communications protocols in seven layers, of which Layer 1 is the physical layer which is concerned with the physical interfaces between devices and the communications medium, and Layer 2 is the data link layer which is concerned with sending and receiving blocks of data together with information for example for synchronization and error and flow control. For LANs, the data link layer is generally considered as comprising two sub-layers, referred to as the LLC (logical link control) layer and the MAC (medium access control) layer. The LLC layer (Layer 2) is addressed by IEEE Standard 802.2. The CSMA/CD Standards address communications at the MAC and physical layers (Layers 2 and 1).
A particularly convenient and popular physical medium for LAN communications is twisted pair wiring as is commonly used for telephone communications. Such wiring typically consists of 0.4 mm to 0.6 mm diameter (26 AWG to 22 AWG) unshielded wires twisted together in pairs in a multipair cable. For example, one of the options for the physical layer documented for CSMA/CD is referred to as 10BASE-T and provides baseband communications at a data rate of 10 Mb/s over twisted pair wiring. The performance specifications are generally met by up to 100 m (meters) of 0.5 mm telephone twisted pair wire without the use of a repeater. Longer wiring lengths are permitted as long as the performance specifications, in particular a maximum delay, are met.
Accordingly, devices that are located relatively close to one another, for example within a building, can be relatively easily connected in a LAN using twisted pair wiring. For CSMA/CD communications via the LAN and for access to the Network, each device is easily equipped with an Ethernet interface card, which is connected via a respective twisted pair of wires to a repeater or CSMA/CD hub, and with TCP/IP (Transmission Control Protocol/Internet Protocol) software that handles the packetized communications at Layers 3 and 4 of the OSI model (Network and Transport Layers, respectively).
Increasingly, access to the Network is required from devices that are relatively distant from existing Network facilities. For example, such devices may be located within residences and small businesses, and they may be isolated computers or they may be connected in a LAN that is not connected to the rest of the Network. Such devices may for example comprise general-purpose computers or specific-purpose devices such as a Network browser, game machine, and/or entertainment device, and may also comprise related and/or ancillary equipment such as workstations, printers, scanners, bridges, routers, etc. that it may be desired to connect to the Network. The generic term xe2x80x9cterminal devicexe2x80x9d and its abbreviation xe2x80x9cTDxe2x80x9d is used below to embrace all such devices.
It is known to provide for access to the Network from a relatively distant terminal device, or TD, via a communications path between a router on the Network and the distant TD, the communications path typically being constituted by a telephone line.
A simple form of such a communications path is a serial link comprising modem communications via a conventional two-wire telephone line. At Layers 1 and 2 of the OSI model the CSMA/CD communication, which can not be used on the serial link because of its length and characteristics, is replaced for the communication with each distant TD by modem communications via the respective telephone line and a point-to-point protocol, such as PPP (Point to Point Protocol) or SLIP (Serial Link Internet Protocol). Currently, modem communications generally provide a maximum data rate of 28.8 kb/s, and may typically operate in practice at lower, fall-back, data rates such as 19.2 or 14.4 kb/s. Such data rates are increasingly insufficient to meet demands imposed on communications for Network access, in particular for rapid downloading of relatively large amounts of data, e.g. for graphics. In addition, use of such modem communications prevents simultaneous use of the telephone line for telephone communications. Furthermore, such a communications path is set up as a dialled connection via the public switched telephone network (PSTN), which involves the inconvenience to the distant TD user of having to establish the dialled connection and the disadvantage of long connection times via the PSTN.
An alternative form of telephone communications path comprises an ISDN (Integrated Services Digital Network) telephone line. This provides two 64 kb/s B-channels each of which can be used for carrying voice communications or data A TD can be connected to the ISDN line via a terminal adapter, which can thereby provide a total bit rate of 128 kb/s for data on both B-channels, or 64 kb/s for data on one B-channel simultaneously with digital telephone voice communications on the other B-channel. While this provides a significant increase in data rate compared with using a conventional two-wire telephone line, it requires an ISDN telephone line which, in the relatively limited areas in which it is available, involves additional cost, and the connection still has the disadvantage of being a dialled connection via the telephone network. In addition, a terminal adapter is generally more costly than a modem. Furthermore, even data rates of 64 kb/s or 128 kb/s are likely to be increasingly insufficient with evolution of the Network.
Higher speed telecommunications lines may be available for lease to provide high data rate communications, but these are not economical for TDs in residences and most small businesses. Cable modems have also been proposed for providing Network access via coaxial (coax) or hybrid fiber-coax (HFC) cable television distribution networks that provide bidirectional communications. While such proposals offer the possibility of high data rates, they are also limited to their own serving areas and are likely to involve relatively high costs for both the modem equipment and the ongoing use of the service.
Accordingly, there is an increasing need to facilitate access from terminal devices to the Network at relatively low cost both for equipment and ongoing service, that is not restricted to particular areas, that provides for high data rates, and that desirably does not preempt telephone communications or require long connection times via the PSTN. An object of this invention is to address this need.
According to one aspect, this invention provides a method of communicating information packets to and from a CSMA/CD (Carrier Sense Multiple Access with Collision Detection) path via a bidirectional communications path, comprising the steps of coupling a first end of the communications path to a first modem; communicating information packets between the CSMA/CD path and the first modem via a CSMA/CD interface: coupling a second end of the communications path to a second modem; communicating information packets and control information, for controlling operation of the second modem, from the first modem to the second modem via the communications path; and communicating information packets from the second modem to the first modem via the communications path under control of the control information; the control information providing half duplex communications on the bidirectional communications path.
The half duplex communications, which can alternatively be considered as time division duplex or time compression multiplex communications, avoid collisions or interference between information packets communicated in the two directions of communication on the communications path by ensuring that the communications in the two directions take place at different times.
Preferably each step of communicating information packets comprises enveloping them in information frames which also comprise error check fields for error checking of at least the enveloped information packets. Information relating to operation of the modems, such as the control information from the first modem to the second modem and response information from the second modem to the first modem, can be included in at least some of the information frames and/or in further frames which comprise this information and an error check field for error checking of at least this information.
Preferably each information packet communicated between the modems via the communications path comprises at least address, length, and data fields of an Ethernet frame communicated via the CSMA/CD path, it desirably also comprises a frame check sequence of the respective Ethernet frame, and may further comprise a preamble and start frame delimiter of an Ethernet frame.
Advantageously, the communications path comprises a two-wire telephone subscriber line and the modems communicate said information packets via the line at frequencies greater than telephone signal frequencies, the method further comprising the steps of communicating telephone signals via the line and, at each end of the line, combining telephone signals and information packets to be communicated via the line, and separating telephone signals and information packets communicated via the line, using a diplexer.
The method can further comprise the step of multiplexing signals of the first modem for communicating information packets between the first modem and a plurality of second modems.
The method can further comprise the steps of monitoring errors in communicating said information packets between the first and second modems via the communications path, and determining operations of the first and second modems in dependence upon monitored errors. The step of determining operations of the moderns in dependence upon monitored errors can comprise varying a signal bandwidth and/or a modulation method of the modems. This enables an optimum rate to be achieved for communicating information packets via any particular two-wire line.
The method can further comprise the step of communicating information packets between the second modem and a second CSMA/CD path via a second CSMA/CD interface.